Highly enhanced adsorption of antibiotics on aged polyamide microplastics

Microplastics in the environment typically undergo aging processes, which could change their physicochemical properties, thereby affecting their adsorption capacities for contaminants. In this study, the effects of aging on the physicochemical properties of polyamide (PA) and the adsorption behavior of ciprofloxacin (CIP) and trimethoprim (TMP) were investigated. After aging, the specific surface area, crystallinity, hydrophobicity, negative surface charge, and oxygen-containing functional groups of the aged PA increased. The adsorbed quantities of CIP and TMP on the aged PA sharply increased 3.90–13.3 times and 24.5–48.5 times, respectively, when compared with that on the pristine PA, and had positive relationship with aging time. This was attributed to the increase in specific surface area and oxygen-containing functional groups of PA with the aging time. Increased oxygen-containing functional groups promoted the formation of hydrogen bonds between antibiotics and aged PA. Adsorption of CIP and TMP on aged PA was negatively correlated with the ionic strength, depending on ion types. The inhibitory effects of ionic strength on the adsorption of CIP and TMP increased in the following order: MgCl2 > CaCl2 > NaCl. PA has a strong affinity for neutral CIP and TMP, indicating that hydrophobic interactions are important for the adsorption of CIP and TMP on PA. Even under unfavorable environmental conditions (such as high ionic strength), aged PA adsorbed more antibiotics than pristine PA. This implies that aged PA could notably affect the fate and transport of antibiotics in nature, thereby influencing their bioavailability and ecotoxicity.